The tread wear rating of the tire is a better way to estimate it's longevity than the warranty or other measures in my experience. I believe yours are rated 240 which is low but not exceptionally low. Long life is apparently not a big factor to BMW in choosing the tires. They are not run flats but if you want an all season tire, these are a good buy IMHO:

With a treadwear rating over 500 they should wear more than twice as long.

I am planning to go to a square setup on my 128 very soon when it gets new shoes. I want to be able to rotate tires because the rears wear out so much faster than the fronts. The stability control won't let us burn them so I think it is the camber of the rear tires. The toe in may also be a little high. BMW REALLY wants our cars to understeer, not oversteer.